Finally tonight I guess it was only a matter of time. We've all seen those flash mobs, the swarms of people singing and dancing for a marriage profile or a retirement party. Now they're going corporate with eye popping price tags. Gio benitez explains the evolution of the flash mob. Reporter: Millions of us watched as people spontaneously break into song and dance. But what was once just fun is now a cash cow. Companies are paying for spontaneity. Here's a flash mob in times square paid for by a hair care line. And here's another for a corporate seminar, even surprising rudy giuliani. That's a flash, flash, flash mob? Reporter: And I mean we're talking about big clients here paying big money just for a flash mob. Yes, we do. Reporter: Jackie is a producer for flash mob america. The biggest flash mob probably was last summer, and it was for a corporate client and we had about 700 people. Reporter: And it can cost anywhere from $1000 to 80 grand. Flash mob america also works for individuals. This man is paying for a flash mob to surprise his sweetheart. 250 volunteers recruited online to join the mob. We figured if a man is going to go through this much to make it special for the person that he wants to spend the rest of his life with, we might as well help him and be part of it. Reporter: They rehearsed for three hours and then headed out to new york's central park. The couple is here. The boyfriend is clearly nervous and the girlfriend has no idea she's about to become a bride to be. After minutes, the dancing begins. The mob grows, surprising the girlfriend and taking her breath away. She just said yes. I'm overwhelmed right now, completely overwhelmed. Reporter: Flash mob producers cashing in on a magical moment. Gio benitez, abc news, new york. Worth every penny.

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